Wavelength-dependent optical detectors are essential optical components that are incorporated in a myriad of applications including spectrometers, optical interconnects and optical communications systems.
An existing wavelength-dependent optical detector is the so-called metal-semiconductor-metal (MSM) photodetector. In this device, an interdigitated pair of metal electrodes is deposited on a surface of a semiconductor. Light illuminating the MSM device is absorbed in the semiconductor producing charge carriers that drift to the neighboring metal electrodes when a voltage is applied to the metal electrodes. The resulting light-induced current is amplified and detected by an amplifier. The wavelength-dependence of the MSM device is partially determined by the absorption characteristics of the semiconductor in the MSM device. GaAs is used as the semiconductor for MSM devices in the 800 nm wavelength range. InAlAs deposited on InGaAs is used as the semiconductor for MSM devices in the 1600 nm wavelength range. The prior art teaches that the wavelength-dependence of the MSM device can be further selected by creating a standing wave on the MSM detector and fabricating the MSM device such that metal electrodes have a particular spacing, for example, a quarter of the wavelength of light to be detected.
While such MSM devices have been successfully employed in a variety of applications, a principal limitation of the MSM device is that the wavelength-dependence cannot be dynamically tuned. It is manifest that this is also the case for other optical detectors that are not wavelength-dependent, such as photodiodes and photomultiplier tubes. Prior art solutions to this technical challenge include external means for dynamically tuning the wavelength of light detected. Solutions include monochromators, interferometers, multiplexers/demultiplexers, spatial optical filters, spectral optical filters (including cavity resonators) and diffraction gratings. For example, see U.S. Pat. Nos. 6,583,900, 6,594,410 and 6,597,841. However, the speed with which the selected wavelength can be changed in these approaches is limited when the dynamic tuning is based on mechanical motion, such as that associated with a stepper motor or thermal expansion. This is also the case when the dynamic tuning is based on the propagation of waves (for example, sound) in a medium, such as in an acousto-optic modulator or a dynamic diffraction grating. The response time for dynamic tuning of the wavelength-dependence of the existing optical detectors in conjunction with such external means is substantially longer than a microsecond and is typically hundreds to thousands of microseconds. A PIN detector with multiple quantum wells can be dynamically tuned with a fast response time; however, such devices only have a coarse tuning capability over a small range of wavelengths and require a large biasing voltage. These limitations in the dynamic tuning of the wavelength dependence of existing optical detectors are particularly problematic in existing or proposed optical communications systems based on Wavelength Division Multiplexing (WDM).
In optical communications systems based on WDM, a combination of time dependent multiplexing (interleaved packets of information), frequency dependent multiplexing (information communicated using multiple, different wavelengths) and/or spread spectrum (wideband) encoding techniques such as code division multiple access are used. Systems include coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM). Recent proposals include 80 channels utilizing a wavelength range centered around 1550 nm (193,300 GHz) with a channel spacing of approximately 0.4 nm (50 GHz) and optical packets of information spaced on time scales on the order of nanoseconds. Future systems will employ more channels (smaller channel spacing) and packets of information spaced on shorter time scales.
To be useful in detecting packets of information based on wavelength in a WDM system, it is highly desirable to be able to switch the wavelength dependence of the optical detector on times scales on the order of or less than the length of the optical packets of information. This necessitates response times for dynamic tuning of the wavelength dependence of the optical detector of a few nanoseconds or less. Response times of this order are well beyond the capability of most of the existing solutions. The alternative, involving a plurality of wavelength-dependent optical detectors with slow dynamic tuning response times, would be expensive and difficult to manufacture and maintain. Each wavelength in the optical system would require a separate detector, the related electronics for amplifying detected signals, as well as a fixed optical filter capable of resolving the small band of wavelengths corresponding to the channel spacing. For example, see U.S. Pat. Nos. 5,546,209, 5,910,851, 6,307,660 and 6,556,321.
As a consequence, there is a need for a wavelength-dependent optical detector that can be dynamically tuned with a response time less than a few nanoseconds for WDM applications, and more generally with a response time less than a microsecond for other applications. It would also be advantageous if the wavelength-dependent optical detector could be dynamically tuned to resolve the narrow channel spacing in WDM systems yet have a wide tuning range. Furthermore, it would be advantageous if such a wavelength-dependent optical detector with fast dynamic tuning were electronically controlled using a low voltage thereby allowing ease of integration with other components.